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In the bowels of Europe’s biggest prison 10 years ago, a burly armed robber in his early twenties found himself consigned to the cell below a seasoned jihadi. The younger man had grown up in an unforgiving Paris suburb not far from the prison, another disenchanted young Muslim languishing on the fringes of the French capital.

“I knew a lot of criminals in jail . . . robbers, drug-traffickers,” the young man, of Malian descent, would tell French police under questioning years later. He met terrorists, too. But only one, he said, made a lasting impression. The Algerian-born jihadi one floor up, 17 years his senior, was supposedly in isolation but the two managed to communicate. The older man had been in Afghanistan and said he had been tortured by western agencies. “The only reason I stayed in contact with him [after they were both released] was not because of religion,” the younger man said. “It was because, as humans, he was someone who touched me.”

The young man was Amédy Coulibaly. On January 9 he killed four Jewish hostages at a kosher supermarket in Paris. He died in a hail of bullets when police stormed the building. The jihadi in the cell above was Djamel Beghal, jailed in 2001 for plotting to blow up the US embassy in Paris. He also cast his influence over another inmate whose stint in the vast Fleury-Mérogis prison on Paris’s southern fringe coincided with Coulibaly’s: Chérif Kouachi, who, accompanied by his brother Said, massacred the staff of Charlie Hebdo, a satirical newspaper that had printed cartoons of the Prophet Mohammed.

“Beghal was the real mentor, a scholar who taught them about religion, about jihad,” says Jean-Charles Brisard, a former counsel to France’s chief antiterrorism prosecutor and an expert on al-Qaeda who now advises governments on counter-terrorism. “The time in prison was crucial.”

Beghal, through his lawyer, has denied any involvement in the Paris attacks.

But the role of one of France’s most notorious prisons — built to hold 2,855 inmates and now home to more than 4,000 — in the long genesis of the Paris attacks has redoubled concerns that Europe’s jails are serving as recruitment centres for violent Islamists.

As European Muslims, either captured as they embark for the wars in Syria and Iraq or as they return, swell the ranks of prisoners with jihadi leanings, the challenge of countering radicalisation in prisons becomes ever more pressing.

Simply consigning thousands of returning fighters to jail would be “an invitation to radicalisation”, warns Gilles de Kerchove, the EU’s top counter-terrorism official.

In the wake of the Paris attacks, Manuel Valls, France’s prime minister, announced significant increases in intelligence-gathering, surveillance and attempts to counter radicalisation.

One option being looked at in France is to separate radical Islamist prisoners from the rest of the prison population to try and protect impressionable inmates from indoctrination. But experts remain divided over the extent of radicalisation in prisons and what to do about it. Much of the evidence is anecdotal and, for the moment at least, the numbers involved are relatively small — scarcely a few hundred in most western countries.

Terror cells within cells

“Isolating them [the Islamist prisoners] is a problem,” says Louis Caprioli, a former French intelligence counter-terrorism chief.

“It forms a terrorist cell within a prison. They come out even more radicalised. It’s a solution to prevent contamination but once they come out you absolutely have to have them under surveillance.”

All three of the Paris attackers had been monitored at different stages after their release from detention. But surveillance is a mammoth task, given the resources required to monitor hundreds of former inmates — possibly far more, depending on how widely the authorities define “radical”.

The alternative — with which the UK and other governments have dabbled — is to try to avoid segregation, in the hope that mingling with other inmates might lead convicted Islamist terrorists and plotters to disavow violence.

“Both schools of thought are failing,” says Haras Rafiq, a former adviser to the UK government on combating extremism and now managing director of the Quilliam Foundation, a think-tank that tries to do the same. He warns that the need for protection can drive inmates into the arms of self-appointed “emirs” at the head of prison gangs.

Mr Rafiq tells the story of a lapsed British Muslim sentenced to a few months in prison after a brawl. It was his first time inside. Previously, he neither prayed nor fasted. “He went into prison very afraid. He started to explore his faith for answers and to become a better person,” says Mr Rafiq. “He got straight in with the Islamist gang. They started very political, racial indoctrination: ‘Brother, you are innocent, it’s really the corrupt judicial system that sent you in for being a Muslim. The only way is to be part of our gang.’ They gave him a lot of literature that is not necessarily Islamist but feeds the idea of ‘them’ and ‘us’. This is the first point of recruitment: them and us.”

The man came out of prison an Islamist. But, Mr Rafiq says, “he didn’t realise he had been radicalised”. He would gradually return to more moderate views — but others might not.

Alyas Karmani, an imam and community activist, told a British parliamentary inquiry into radicalisation in 2011 of a man awaiting trial in London’s Belmarsh high-security prison. His cell was three along from that of Abdullah al-Faisal, a preacher convicted in 2003 for soliciting the murder of Jews, Americans, Hindus and Christians. “Within three days, Abdullah al-Faisal had convinced him to undertake a martyrdom mission,” the inquiry was told. After leaving jail he went directly to Yemen and was only dissuaded from carrying out an attack by local scholars who changed his mind.

Violent minority

Experts stress that, just as Islamists represent a minority of Muslims, so those committed to violence in pursuit of their aims represent a minority of Islamists — and only a handful of those pass through prison. Many more are increasingly radicalised online. However, Mr Brisard says those who have embraced extremism in prison tend to have “more dangerous profiles” than those who did so while at liberty.

Mohammed Merah and Mehdi Nemmouche, who murdered Jews respectively in 2012 in Toulouse and last year in Brussels, had both passed through French prisons after robbery convictions. Richard Reid, who tried to detonate a bomb in his shoe on a flight to Miami in 2001, had converted to Islam while serving a sentence for theft in a British jail. At least five of the 134 people convicted of Islamism-related terrorist offences in the UK between 1999 and 2010 had served prison terms, according to research by the Henry Jackson Society think-tank in London. They include Muktar Said Ibrahim, a member of the cell behind attempted bombings two weeks after the July 7 2005 attacks in London. Draft UK counter-terrorism legislation, drawing on an existing “deradicalisation” strategy, contains measures to monitor prisoners at risk of indoctrination.

Experts say conditions in French prisons add to the climate for indoctrination. France’s secular traditions prohibit gathering information on the religious persuasion of prisoners but experts suggest that at least half the occupants of French prisons are Muslim, higher still in urban areas. That is at least five times more than the Muslim share of the population. The French prison system suffers more inmate suicides than any other European country, according to the Council of Europe.

Among French Muslims accustomed, in the words of academic Farhad Khosrokhavar, to being regarded by their countrymen as “insects”, the road to violent Islamism does not begin with Islam but with hatred. “People think it’s the other way round,” says Mr Khosrokhavar, a sociologist at the School for Advanced Studies in Social Sciences in Paris, who has spent two decades analysing Islam in France. “But radicalisation comes first: ‘I am despised; I hate them.’”

In the course of hundreds of interviews for a forthcoming book, Mr Khosrokhavar found that young French Muslims in deprived areas who embrace the notion of jihad — or holy war — do so before they have grasped even the most basic tenets of Islam. “Jihad is the only tool they have to give sacred meaning to their hatred of society. Once they become jihadised, they learn Islam — and mostly they do it in prison.”

Chérif Kouachi had already espoused jihadism by the time he arrived at Fleury-Mérogis prison in 2005 (though he told investigators that he had been in two minds about his intended mission to fight US forces in Iraq, which had been foiled by his arrest). Coulibaly first saw the inside of a cell at 17 and went on to clock up jail time for theft, violence, armed robbery and drug trafficking.

The French justice ministry has claimed that Coulibaly, Chérif Kouachi and Djamel Beghal could not have spent time together in prison because of the dates of their terms and the fact that Beghal was in isolation. But Coulibaly’s interrogation records, seen by the Financial Times, include him recounting meetings with Beghal while a police intercept has Beghal describing Kouachi in prison. However briefly they coincided, it was long enough for Coulibaly to form a bond with Beghal.

Syria and Iraq

After he got out in 2007, Coulibaly made frequent trips to the hilly region of Cantal in southern France, where Beghal took up residence under house arrest after his own release. Police surveillance records document a masterclass in indoctrination by a man who, during a stint in the UK, had been a devotee of Abu Hamza, the radical cleric of Finsbury Park mosque, and Abu Qatada. An early psychological assessment of Coulibaly noted “immature and psychopathic personality traits” and concluded that “the search for power” underpinned his every action. He was putty in Beghal’s hands.

“Djamal Beghal is a dazzling man,” says Mr Caprioli, on whose watch Beghal was captured. “He has charisma. He’s on another level intellectually from these boys.”

In 2010, Beghal, Coulibaly and Chérif Kouachi were arresTed in connection with a plot to free a convicted terrorist from a French prison. While Kouachi was released for lack of evidence Beghal and Coulibaly were convicted. Coulibaly was released early last year; Beghal remains behind bars.

Europe’s authorities are once again trying to choke off the pipeline that leads from prison to jihadism. “Deradicalisation” programmes in Singapore, Saudi Arabia and elsewhere offer one option. Mr Valls says he will double the budget for imams working in prisons and add 60 more to the 182 who currently visit inmates.

But that remains a tiny number compared with an estimated 35,000 Muslims in French prisons. And counter-terrorism experts warn that extremists in prison have responded to increased scrutiny by switching from public proselytising to more clandestine forms of indoctrination.

As a new generation of jihadis return from Syria and Iraq — many of them from more affluent backgrounds than the likes of Coulibaly or the Kouachi brothers and strangers to prison — there are fears that tough criminal sentences could make confirmed Islamists of those who come back traumatised or disillusioned.

Mr de Kerchove says that those “with blood on their hands” must face criminal justice but he also calls for rehabilitation programmes, warning on the dangers of indiscriminately jailing all those who return from waging jihad.

“People are scared and say they want to see them all thrown in jail,” says Mr de Kerchove. “But it is not the right approach.”